Method of manufacturing a semiconductor device and a semiconductor device

ABSTRACT

In a method of manufacturing a semiconductor device, a fin structure, in which first semiconductor layers and second semiconductor layers are alternately stacked, is formed. A sacrificial gate structure is formed over the fin structure. The first semiconductor layers, the second semiconductor layer and an upper portion of the fin structure at a source/drain region of the fin structure, which is not covered by the sacrificial gate structure, are etched. A dielectric layer is formed over the etched upper portion of the fin structure. A source/drain epitaxial layer is formed. The source/drain epitaxial layer is connected to ends of the second semiconductor wires, and a bottom of the source/drain epitaxial layer is separated from the fin structure by the dielectric layer.

This application claims priority of Provisional Application No.62/552,895 filed on Aug. 31, 2017, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The disclosure relates to method of manufacturing semiconductorintegrated circuits, and more particularly to method of manufacturingsemiconductor devices including fin field effect transistors (FinFETs)and/or gate-all-around (GAA) FETs, and semiconductor devices.

BACKGROUND

As the semiconductor industry has progressed into nanometer technologyprocess nodes in pursuit of higher device density, higher performance,and lower costs, challenges from both fabrication and design issues haveresulted in the development of three-dimensional designs, such as amulti-gate field effect transistor (FET), including a fin FET (FinFET)and a gate-all-around (GAA) FET. In a Fin FET, a gate electrode isadjacent to three side surfaces of a channel region with a gatedielectric layer interposed therebetween. Because the gate structuresurrounds (wraps) the fin on three surfaces, the transistor essentiallyhas three gates controlling the current through the fin or channelregion. Unfortunately, the fourth side, the bottom part of the channelis far away from the gate electrode and thus is not under close gatecontrol. In contrast, in a GAA FET, all side surfaces of the channelregion are surrounded by the gate electrode, which allows for fullerdepletion in the channel region and results in less short-channeleffects due to steeper sub-threshold current swing (SS) and smallerdrain induced barrier lowering (DIBL). As transistor dimensions arecontinually scaled down to sub 10-15 nm technology nodes, furtherimprovements of the GAA FET are required.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1A shows a perspective view, and FIGS. 1B and 1C show crosssectional views of a semiconductor FET device according to an embodimentof the present disclosure. FIGS. 1D and 1E show enlarged cross sectionalviews corresponding to AR1 of FIGS. 1A and 1B, and FIG. 1F shows anenlarged cross sectional view corresponding to AR2 of FIGS. 1A and 1C.

FIG. 2A shows a perspective view, and FIGS. 2B and 2C show crosssectional views of a semiconductor FET device according to an embodimentof the present disclosure. FIGS. 2D and 2E show enlarged cross sectionalviews corresponding to AR1 of FIGS. 2A and 2B, and FIG. 2F shows anenlarged cross sectional view corresponding to AR2 of FIGS. 2A and 2C.

FIG. 3A shows a perspective view, and FIGS. 3B and 3C show crosssectional views of a semiconductor FET device according to an embodimentof the present disclosure. FIG. 3D shows an enlarged cross sectionalview corresponding to AR1 of FIGS. 3A and 3B, and FIG. 3E shows anenlarged cross sectional view corresponding to AR2 of FIGS. 3A and 3C.

FIG. 4A shows a perspective view, and FIGS. 4B and 4C show crosssectional views of a semiconductor FET device according to an embodimentof the present disclosure. FIG. 4D shows an enlarged cross sectionalview corresponding to AR1 of FIGS. 4A and 4B, and FIG. 4E shows anenlarged cross sectional view corresponding to AR2 of FIGS. 4A and 4C.

FIG. 5 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 6 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 7 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 8 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIGS. 9A, 9B and 9C show one of the various stages of manufacturing asemiconductor FET device according to an embodiment of the presentdisclosure.

FIGS. 10A, 10B and 10C show one of the various stages of manufacturing asemiconductor FET device according to an embodiment of the presentdisclosure.

FIGS. 11A, 11B and 11C show one of the various stages of manufacturing asemiconductor FET device according to an embodiment of the presentdisclosure.

FIGS. 12A, 12B and 12C show one of the various stages of manufacturing asemiconductor FET device according to an embodiment of the presentdisclosure.

FIGS. 13A, 13B and 13C show one of the various stages of manufacturing asemiconductor FET device according to an embodiment of the presentdisclosure.

FIGS. 14A, 14B and 14C show one of the various stages of manufacturing asemiconductor FET device according to an embodiment of the presentdisclosure.

FIGS. 15A, 15B and 15C show one of the various stages of manufacturing asemiconductor FET device according to an embodiment of the presentdisclosure.

FIGS. 16A, 16B and 16C show one of the various stages of manufacturing asemiconductor FET device according to an embodiment of the presentdisclosure.

FIGS. 17A, 17B and 17C show one of the various stages of manufacturing asemiconductor FET device according to an embodiment of the presentdisclosure.

FIGS. 18A, 18B and 18C show one of the various stages of manufacturing asemiconductor FET device according to an embodiment of the presentdisclosure.

FIGS. 19A, 19B and 19C show one of the various stages of manufacturing asemiconductor FET device according to an embodiment of the presentdisclosure.

FIGS. 20A, 20B and 20C show one of the various stages of manufacturing asemiconductor FET device according to an embodiment of the presentdisclosure. FIG. 20D shows one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIG. 21 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 22 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 23 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 24 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIGS. 25A, 25B and 25C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 26A, 26B and 26C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 27A, 27B and 27C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 28A, 28B and 28C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 29A, 29B and 29C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 30A, 30B and 30C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 31A, 31B and 31C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 32A, 32B and 32C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 33A, 33B and 33C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 34A, 34B and 34C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 35A, 35B and 35C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 36A, 36B and 36C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 37A, 37B and 37C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 38A, 38B and 38C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 39A, 39B and 39C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 40A, 40B and 40C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 41A, 41B and 41C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 42A, 42B and 42C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 43A, 43B and 43C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

FIGS. 44A, 44B and 44C show one of the various stages of manufacturing asemiconductor FET device according to another embodiment of the presentdisclosure.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the invention. Specific embodiments or examples of components andarrangements are described below to simplify the present disclosure.These are, of course, merely examples and are not intended to belimiting. For example, dimensions of elements are not limited to thedisclosed range or values, but may depend upon process conditions and/ordesired properties of the device. Moreover, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed interposing the first and second features, suchthat the first and second features may not be in direct contact. Variousfeatures may be arbitrarily drawn in different scales for simplicity andclarity.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. In addition, the term“being made of” may mean either “comprising” or “consisting of.” In thepresent disclosure, a phrase “one of A, B and C” means “A, B and/or C”(A; B; C; A and B; A and C; B and C; or A, B and C), and does not meanone element from A, one element from B and one element from C, unlessotherwise described.

A gate-all-around FET (GAA-FET) generally includes one or moresemiconductor wires having a channel region and source/drain regionsdisposed on both ends of the channel region. To manufacture thesemiconductor wire(s), stacked layers of different semiconductormaterials, one(s) of which is/are a sacrificial layer, are formed, andthen the sacrificial layer(s) is/are removed, thereby leavingsemiconductor wire(s). In the source/drain regions, a source/drainepitaxial layer is formed to be connected to the channel regions(semiconductor wires). In such a structure, suppressing a leakagecurrent from the source/drain epitaxial layer to the fin structure is animportant issue for transistor performances. In some cases, anti-punchthrough (APT) implantation to the fin structure is used to decrease theleakage current. However, APT impurities may diffuse into the channelregion by subsequent thermal processes, which impacts channel mobilityand degrades transistor performance.

The present disclosure provides a semiconductor device, such as a GAAFET, which can reduce the leakage current. In this disclosure, asource/drain refers to a source and/or a drain. It is noted that in thepresent disclosure, a source and a drain are interchangeably used andthe structures thereof are substantially the same, unless otherwisedescribed.

FIG. 1A shows a perspective view, and FIGS. 1B and 1C show crosssectional views of an n-type GAA FET device according to an embodimentof the present disclosure.

As shown in FIG. 1, a GAA FET is disposed over a substrate 10. In theGAA FET, semiconductor wires 25 are provided over the semiconductorsubstrate 10, and vertically arranged along the Z direction (the normaldirection of the principal surface of the substrate 10). In someembodiments, the substrate 10 includes a single crystallinesemiconductor layer on at least it surface portion. The substrate 10 maycomprise a single crystalline semiconductor material such as, but notlimited to Si, Ge, SiGe, GaAs, InSb, GaP, GaSb, InAlAs, InGaAs, GaSbP,GaAsSb and InP. In certain embodiments, the substrate 10 is made ofcrystalline Si.

The substrate 10 may include in its surface region, one or more bufferlayers (not shown). The buffer layers can serve to gradually change thelattice constant from that of the substrate to that of the source/drainregions. The buffer layers may be formed from epitaxially grown singlecrystalline semiconductor materials such as, but not limited to Si, Ge,GeSn, SiGe, GaAs, InSb, GaP, GaSb, InAlAs, InGaAs, GaSbP, GaAsSb, GaN,GaP, and InP. In a particular embodiment, the substrate 10 comprisessilicon germanium (SiGe) buffer layers epitaxially grown on the siliconsubstrate 10. The germanium concentration of the SiGe buffer layers mayincrease from 30 atomic % germanium for the bottom-most buffer layer to70 atomic % germanium for the top-most buffer layer.

In the GAA FET, the semiconductor wires 25, which are channel layers,are disposed over the substrate 10. In some embodiments, thesemiconductor wires 25 are made of Si. In other embodiments, thesemiconductor wires 25 are made of Si_(1-x)Ge_(x), where 0≤x≤0.3. Insome embodiments, the semiconductor wires 25 are disposed over a finstructure 11 protruding from the substrate 10. The fin structure 11 isembedded in an isolation insulating layer 15. The fin structure 11 iscovered by a first fin liner layer 13A and a second fin liner layer 13Bdisposed over the first fin liner layer 13A. The fin liner layers aremade of silicon oxide, SiN or a silicon nitride-based material (e.g.,SiON, SiCN or SiOCN).

Each of the channel layers 25 is wrapped around by a gate dielectriclayer 102 and a gate electrode layer 104. In some embodiments, the gatedielectric layer 102 includes an interfacial layer 102A and a high-kdielectric layer 102B. The gate structure includes the gate dielectriclayer 102, the gate electrode layer 104 and sidewall spacers 32.Although FIG. 1 shows five semiconductor wires 25, the number of thesemiconductor wires 25 is not limited to five, and may be as small asone, or more than five and may be up to fifteen (15).

In certain embodiments of the present disclosure, one or more workfunction adjustment layers 104A and 104B are interposed between the gatedielectric layer 102 and a metal gate electrode layer 104C.

In the GAA FET, a source/drain epitaxial layer 50 is disposed over thefin structure 11. The source/drain epitaxial layer 50 includes one ormore layers of Si, SiP, SiC and SiCP for an n-channel FET. Thesource/drain epitaxial layer 50 is in direct contact with the channellayer 25, and is separated by a dielectric layer 35 as inner spacers andthe gate dielectric layer 102 from the gate electrode layer 104. Thedielectric layer 35 is made of a low-k (dielectric constant lower thanthe dielectric constant of SiO₂) material in some embodiments. The low-kmaterial includes SiOC, SiOCN, organic material or porous material, orany other suitable material. In other embodiments, the dielectric layer35 is made of silicon oxide and/or silicon nitride, or any othersuitable dielectric material.

Further, a contact etch stop layer (CESL) 60 is formed over thesource/drain epitaxial layer 50 and sidewall spacers 32, and aninterlayer dielectric (ILD) layer 80 is disposed over the CESL 60. TheCESL 60 is made of SiN or a silicon nitride-based material (e.g., SiON,SiCN or SiOCN). The materials for the ILD layer 80 include compoundscomprising Si, O, C and/or H, such as silicon oxide, SiCOH and SiOC.Organic materials, such as polymers, may be used for the ILD layer 80.

In the present disclosure, as shown in FIGS. 1A-1C, the semiconductorwires 25 and the source/drain epitaxial layer 50 are disposed above thefin structure 11, and a bottom of the source/drain epitaxial layer 50 isseparated from the fin structure 11 by the dielectric layer 35. Further,the same dielectric layer 35 is utilized as an inner spacer between thesource/drain epitaxial layer 50 and the gate electrode 104.

Under the source/drain epitaxial layer 50, the upper portion of the finstructure 11 is recessed to have a groove, in which the dielectric layer35 is disposed. In the present disclosure, the groove has a uniquestructure.

FIGS. 1D and 1E show enlarged cross sectional views of the groove alongthe X direction (source to drain direction) corresponding to AR1 ofFIGS. 1A and 1B and FIG. 1F shows an enlarged cross sectional view ofthe groove along the Y direction (gate extending direction)corresponding to AR2 of FIGS. 1A and 1C.

As explained below with reference to FIGS. 5-20C, the groove 18 has aV-shape groove, and in particular has a “double etched profile” causedby two dry etching operations performed at different process steps, asshown in FIGS. 1D and 1E. In some embodiments, as shown in FIG. 1D, thedielectric layer 35 filled in the groove 18 has a height (highestposition measured from the bottom of the groove), which corresponds tothe interface between the lowest-positioned gate electrode 104 and thelowest-positioned semiconductor wire 25. In other embodiments, thedielectric layer 35 filled in the groove 18 has the height, whichcorresponds to the bottom of the lowest-positioned gate electrode 104 orlower, as shown in FIG. 1E. The height of the dielectric layer 35 canvary between the cases of FIGS. 1D and 1E. The dielectric layer 35 isconnected to the inner spacer formed at the end of the lowest-positionedgate electrode 104.

As shown in FIG. 1E, the groove has a depth D11, which is in a rangefrom about 5 nm to about 50 nm in some embodiments, a top width W11,which is in a range from about 10 nm to about 50 nm in some embodiments,and a bottom width W12, which is in a range from about 1 nm to about 10nm in some embodiments, all in the X direction. In FIG. 1E, the groove18 has a step at the middle thereof. The location of the step D12measured from the top of the groove is in a range from about 2 nm toabout 40 nm in some embodiments, and the width W13 at the step is in arange from about 5 nm to about 25 nm in some embodiments. The sidewallof the groove 18 has a wall angle with respect to the Z axis, whichchanges small-large-small from the bottom of the groove. Even when thedielectric layer 35 filled in the groove 18 has a height lower than thebottom of the lowest-positioned gate electrode 104, the distance W14between the gate electrode 104 and the dielectric layer 35 is less thanabout 1 nm. In contrast, in the cross section along the Y direction asshown in FIG. 1F, the groove 18 has a U-shape.

FIG. 2A shows a perspective view, and FIGS. 2B and 2C show crosssectional views of an n-type GAA FET device according to anotherembodiment of the present disclosure. FIGS. 2D and 2E show enlargedcross sectional views corresponding to AR1 of FIGS. 2A and 2B and FIG.2F shows an enlarged cross sectional view corresponding to AR2 of FIGS.2A and 2C. Material, configuration, dimensions and/or processes the sameas or similar to the foregoing embodiments described with respect toFIGS. 1A-1F may be employed in the embodiment of FIGS. 2A-2F, anddetailed explanation thereof may be omitted.

The gate structure and source/drain structures of the n-type GAA FETshown in FIGS. 2A-2C are substantially the same as those of the n-typeGAA FET shown in FIGS. 1A-1C, except for the shape of the groove 18formed at the upper portion of the fin structure 11.

As explained below with reference to FIGS. 25A-30C, the groove 18 isformed by using wet etching and thus has a (111) facet at the sidewallsthereof, as shown in FIGS. 2D and 2E. In some embodiments, as shown inFIG. 2D, the dielectric layer 35 filled in the groove 18 has a height(highest position measured from the bottom of the groove), whichcorresponds to the interface between the lowest-positioned gateelectrode 104 and the lowest-positioned semiconductor wire 25. In otherembodiments, the dielectric layer 35 filled in the groove 18 has theheight, which corresponds to the bottom of the lowest-positioned gateelectrode 104 or lower, as shown in FIG. 2E. The height of thedielectric layer 35 can vary between the cases of FIGS. 2D and 2E. Thedielectric layer 35 is connected to the inner spacer formed at the endof the lowest-positioned gate electrode 104.

As shown in FIG. 2E, the groove has a depth D21, which is in a rangefrom about 5 nm to about 50 nm in some embodiments, a top width W21,which is in a range from about 10 nm to about 50 nm in some embodiments,and a bottom width W22, which is in a range from about 1 nm to about 10nm in some embodiments, all in the X direction. In FIG. 2E, the groove18 has a largest width W23 at the middle in the depth direction of thegroove 18. Thus, the width of the groove increases and then decreasesfrom the bottom to the top. The largest width W23 is in a range fromabout 12 nm to about 60 nm in some embodiments. The location D22 of thelargest width measured from the top of the groove is in a range fromabout 2 nm to about 40 nm in some embodiments. Even when the dielectriclayer 35 filled in the groove 18 has a height lower than the bottom ofthe lowest-positioned gate electrode 104, the distance W24 between thegate electrode 104 and the dielectric layer 35 is less than about 1 nm.As set forth above, when the fin structure is formed from a Si substratehaving (100) principal surface, the bottom-side sidewalls have a (111)facet of Si. In contrast, in the cross section along the Y direction asshown in FIG. 2F, the groove 18 has a U-shape.

FIG. 3A shows a perspective view, and FIGS. 3B and 3C show crosssectional views of a p-type GAA FET device according to anotherembodiment of the present disclosure. FIG. 3D shows an enlarged crosssectional view corresponding to AR1 of FIGS. 3A and 3B and FIG. 3E showsan enlarged cross sectional view corresponding to AR2 of FIGS. 3A and3C. Material, configuration, dimensions and/or processes the same as orsimilar to the foregoing embodiments described with respect to FIGS.1A-2F may be employed in the embodiment of FIGS. 3A-3E, and detailedexplanation thereof may be omitted.

In the p-type GAA FET, the channel layers are formed by semiconductorwires 20, which are made of a semiconductor material different from thesubstrate 10 and/or the fin structure 11. In some embodiments, thesemiconductor wires are made of Si_(1-x)Ge_(x), where 0.2≤x≤0.8. Thesource/drain epitaxial layer 50 includes one or more layers of Si, SiGeor Ge for a p-channel FET.

As explained below with reference to FIGS. 31A-38C, the groove 18 has a“double etched profile” caused by two dry etching operations performedat different process steps, as shown in FIG. 3D. As shown in FIG. 3D,the groove has a depth D31, which is in a range from about 5 nm to about50 nm in some embodiments, a top width W31, which is in a range fromabout 10 nm to about 50 nm in some embodiments, and a bottom width W32,which is in a range from about 1 nm to about 10 nm in some embodiments,all in the X direction.

In FIG. 3D, the groove 18 has a step at the middle thereof. The locationD32 of the step measured from the top of the groove is in a range fromabout 2 nm to about 40 nm in some embodiments, and the width W33 at thestep is in a range from about 5 nm to about 25 nm in some embodiments.The sidewall of the groove 18 has a wall angle with respect to the Zaxis, which changes small-large-small from the bottom of the groove. Insome embodiments, the groove 18 has a largest width W34 near the bottomof the gate electrode layer 104, which is in a range from about 12 nm toabout 60 nm. The distance W35 between the bottom-most channel layer 20and the dielectric layer 35 is less than about 1 nm. In contrast, in thecross section along the Y direction as shown in FIG. 3E, the groove 18has a U-shape.

FIG. 4A shows a perspective view, and FIGS. 4B and 4C show crosssectional views of a p-type GAA FET device according to anotherembodiment of the present disclosure. FIG. 4D shows an enlarged crosssectional view corresponding to AR1 of FIGS. 4A and 4B and FIG. 4E showsan enlarged cross sectional view corresponding to AR2 of FIGS. 4A and4C. Material, configuration, dimensions and/or processes the same as orsimilar to the foregoing embodiments described with respect to FIGS.1A-3E may be employed in the embodiment of FIGS. 4A-4E, and detailedexplanation thereof may be omitted.

The gate structure and source/drain structures of the p-type GAA FETshown in FIGS. 4A-4C are substantially the same as those of the p-typeGAA FET shown in FIGS. 3A-3C, except for the shape of the groove 18formed at the upper portion of the fin structure 11. As explained belowwith reference to FIGS. 39A-44C, the groove 18 is formed by using wetetching and thus has a (111) facet at the sidewalls thereof, as shown inFIGS. 4D and 4E.

As shown in FIG. 4D, the groove has a depth D41, which is in a rangefrom about 5 nm to about 50 nm in some embodiments, a top width W41,which is in a range from about 10 nm to about 50 nm in some embodiments,and a bottom width W42, which is in a range from about 1 nm to about 10nm in some embodiments, all in the X direction. In FIG. 4D, the groove18 has a largest width W43 at the middle in the depth direction of thegroove 18. Thus, the width of the groove increases and then decreasesfrom the bottom to the top. In some embodiments, the width of thegroove, from the bottom to the top, increases, decreases, increases andthen decreases. The largest width W43 is in a range from about 12 nm toabout 60 nm in some embodiments. The location D42 of the largest withmeasured from the top of the groove is in a range from about 2 nm toabout 40 nm in some embodiments. The distance W44 between thebottom-most channel layer 20 and the dielectric layer 35 is less thanabout 1 nm. As set forth above, when the fin structure is formed from aSi substrate having (100) principal surface, the bottom-side sidewallshave a (111) facet of Si. In contrast, in the cross section along the Ydirection as shown in FIG. 4E, the groove 18 has a U-shape.

FIGS. 5-24 show various stages of manufacturing a GAA FET deviceaccording to an embodiment of the present disclosure. It is understoodthat additional operations can be provided before, during, and after theprocesses shown by FIGS. 5-24, and some of the operations describedbelow can be replaced or eliminated, for additional embodiments of themethod. The order of the operations/processes may be interchangeable.Material, configuration, dimensions and/or processes the same as orsimilar to the foregoing embodiments described with respect to FIGS.1A-4E may be employed in the embodiment of FIGS. 5-24, and detailedexplanation thereof may be omitted. In FIGS. 9A-20C, the “A” figures(FIGS. 9A, 10A, . . . ) are perspective views, the “B” figures (FIGS.9B, 10B, . . . ) are cross sectional views cutting a channel regionalong the X direction, and the “C” figures (FIGS. 9C, 10C, . . . ) arecross sectional views at a source/drain region along the Y direction. Inthis embodiment, a manufacturing operation for an n-type GAA FET will beexplained.

As shown in FIG. 5, impurity ions (dopants) 12 are implanted into asilicon substrate 10 to form a well region. The ion implantation isperformed to prevent a punch-through effect. The substrate 10 mayinclude various regions that have been suitably doped with impurities(e.g., p-type or n-type conductivity). The dopants 12 are, for exampleboron (BF₂) for an n-type Fin FET and phosphorus for a p-type Fin FET.

Then, as shown in FIG. 6, stacked semiconductor layers are formed overthe substrate 10. The stacked semiconductor layers include firstsemiconductor layers 20 and second semiconductor layers 25. Further, amask layer 16 is formed over the stacked layers.

The first semiconductor layers 20 and the second semiconductor layers 25are made of materials having different lattice constants, and mayinclude one or more layers of Si, Ge, SiGe, GaAs, InSb, GaP, GaSb,InAlAs, InGaAs, GaSbP, GaAsSb or InP.

In some embodiments, the first semiconductor layers 20 and the secondsemiconductor layers 25 are made of Si, a Si compound, SiGe, Ge or a Gecompound. In one embodiment, the first semiconductor layers 20 areSi_(1-x)Ge_(x), where x is more than about 0.2, or Ge (x=1.0) and thesecond semiconductor layers 25 are Si or Si_(1-y)Ge_(y), where y is lessthan about 0.4, and x>y. In this disclosure, an “M” compound” or an “Mbased compound” means the majority of the compound is M.

In another embodiment, the second semiconductor layers 25 areSi_(1-y)Ge_(y), where y is more than about 0.2, or Ge, and the firstsemiconductor layers 20 are Si or Si_(1-x)Ge_(x), where x is less thanabout 0.4, and x<y. In yet other embodiments, the first semiconductorlayer 20 is made of Si_(1-x)Ge_(x), where x is in a range from about 0.2to about 0.8, and the second semiconductor layer 25 is made ofSi_(1-y)Ge_(y), where y is in a range from about 0.1 to about 0.4.

In FIG. 6, four layers of the first semiconductor layer 20 and fourlayers of the second semiconductor layer 25 are disposed. However, thenumber of the layers are not limited to four, and may be as small as 1(each layer) and in some embodiments, 2-10 layers of each of the firstand second semiconductor layers are formed. By adjusting the numbers ofthe stacked layers, a driving current of the GAA FET device can beadjusted.

The first semiconductor layers 20 and the second semiconductor layers 25are epitaxially formed over the substrate 10. The thickness of the firstsemiconductor layers 20 may be equal to or greater than that of thesecond semiconductor layers 25, and is in a range from about 2 nm toabout 20 nm in some embodiments, and is in a range from about 5 nm toabout 15 nm in other embodiments. The thickness of the secondsemiconductor layers 25 is in a range from about 2 nm to about 20 nm insome embodiments, and is in a range from about 5 nm to about 15 nm inother embodiments. The thickness of each of the first semiconductorlayers 20 may be the same, or may vary.

In some embodiments, the bottom first semiconductor layer (the closestlayer to the substrate 10) is thicker than the remaining firstsemiconductor layers. The thickness of the bottom first semiconductorlayer is in a range from about 10 nm to about 50 nm in some embodiments,or is in a range from 20 nm to 40 nm in other embodiments.

In some embodiments, the mask layer 16 includes a first mask layer 16Aand a second mask layer 16B. The first mask layer 16A is a pad oxidelayer made of a silicon oxide, which can be formed by a thermaloxidation. The second mask layer 16B is made of a silicon nitride (SiN),which is formed by chemical vapor deposition (CVD), including lowpressure CVD (LPCVD) and plasma enhanced CVD (PECVD), physical vapordeposition (PVD), atomic layer deposition (ALD), or other suitableprocess. The mask layer 16 is patterned into a mask pattern by usingpatterning operations including photo-lithography and etching.

Next, as shown in FIG. 7, the stacked layers of the first and secondsemiconductor layers 20, 25 are patterned by using the patterned masklayer 16, thereby the stacked layers are formed into fin structures 29extending in the X direction. In FIG. 7, two fin structures 29 arearranged in the Y direction. But the number of the fin structures is notlimited to two, and may be as small as one and three or more. In someembodiments, one or more dummy fin structures are formed on both sidesof the fin structures 29 to improve pattern fidelity in the patterningoperations. As shown in FIG. 7 the fin structures 29 have upper portionsconstituted by the stacked semiconductor layers 20, 25 and well portions11.

The width W1 of the upper portion of the fin structure along the Ydirection is in a range from about 10 nm to about 40 nm in someembodiments, and is in a range from about 20 nm to about 30 nm in otherembodiments. The height H1 along the Z direction of the fin structure isin a range from about 100 nm to about 200 nm.

The stacked fin structure 29 may be patterned by any suitable method.For example, the structures may be patterned using one or morephotolithography processes, including double-patterning ormulti-patterning processes. Generally, double-patterning ormulti-patterning processes combine photolithography and self-alignedprocesses, allowing patterns to be created that have, for example,pitches smaller than what is otherwise obtainable using a single, directphotolithography process. For example, in one embodiment, a sacrificiallayer is formed over a substrate and patterned using a photolithographyprocess. Spacers are formed alongside the patterned sacrificial layerusing a self-aligned process. The sacrificial layer is then removed, andthe remaining spacers may then be used to pattern the stacked finstructure 29.

After the fin structures 29 are formed, an insulating material layerincluding one or more layers of insulating material is formed over thesubstrate so that the fin structures are fully embedded in theinsulating layer. The insulating material for the insulating layer mayinclude silicon oxide, silicon nitride, silicon oxynitride (SiON),SiOCN, SiCN, fluorine-doped silicate glass (FSG), or a low-k dielectricmaterial, formed by LPCVD (low pressure chemical vapor deposition),plasma-CVD or flowable CVD. An anneal operation may be performed afterthe formation of the insulating layer. Then, a planarization operation,such as a chemical mechanical polishing (CMP) method and/or an etch-backmethod, is performed such that the upper surface of the uppermost secondsemiconductor layer 25 is exposed from the insulating material layer. Insome embodiments, a fin liner layer 13 is formed over the fin structuresbefore forming the insulating material layer. The fin liner layer 13 ismade of SiN or a silicon nitride-based material (e.g., SiON, SiCN orSiOCN).

In some embodiments, the fin liner layers 13 include a first fin linerlayer formed over the substrate 10 and sidewalls of the bottom part ofthe fin structures 11, and a second fin liner layer formed on the firstfin liner layer. Each of the liner layers has a thickness between about1 nm and about 20 nm in some embodiments. In some embodiments, the firstfin liner layer includes silicon oxide and has a thickness between about0.5 nm and about 5 nm, and the second fin liner layer includes siliconnitride and has a thickness between about 0.5 nm and about 5 nm. Theliner layers may be deposited through one or more processes such asphysical vapor deposition (PVD), chemical vapor deposition (CVD), oratomic layer deposition (ALD), although any acceptable process may beutilized.

Then, as shown in FIG. 8, the insulating material layer is recessed toform an isolation insulating layer 15 so that the upper portions of thefin structures 29 are exposed. With this operation, the fin structures29 are separated from each other by the isolation insulating layer 15,which is also called a shallow trench isolation (STI). The isolationinsulating layer 15 may be made of suitable dielectric materials such assilicon oxide, silicon nitride, silicon oxynitride, fluorine-dopedsilicate glass (FSG), low-k dielectrics such as carbon doped oxides,extremely low-k dielectrics such as porous carbon doped silicon dioxide,a polymer such as polyimide, combinations of these, or the like. In someembodiments, the isolation insulating layer 15 is formed through aprocess such as CVD, flowable CVD (FCVD), or a spin-on-glass process,although any acceptable process may be utilized.

In the embodiment shown in FIG. 8, the insulating material layer 15 isrecessed until the upper portion of the fin structure (well layer) 11 isexposed. In other embodiments, the upper portion of the fin structure 11is not exposed. The first semiconductor layers 20 are sacrificial layerswhich are subsequently partially removed, and the second semiconductorlayers 25 are subsequently formed into semiconductor wires as channellayers of an n-type GAA FET. In other embodiments, the secondsemiconductor layers 25 are sacrificial layers which are subsequentlypartially removed, and the first semiconductor layers 20 aresubsequently formed into semiconductor wires as channel layers of ap-type GAA FET.

FIGS. 9A, 9B and 9C show one of the various stages of manufacturing asemiconductor FET device according to an embodiment of the presentdisclosure. After the isolation insulating layer 15 is formed, asacrificial (dummy) gate structure 38 is formed, as shown in FIGS.9A-9C. FIGS. 9A-9C illustrate a structure after a sacrificial gatestructure 38 is patterned over the exposed fin structures 29. Thesacrificial gate structure 38 is formed over a portion of the finstructures which is to be a channel region. The sacrificial gatestructure 38 defines the channel region of the GAA FET. The sacrificialgate structure 38 includes a sacrificial gate dielectric layer 31 and asacrificial gate electrode layer 30. The sacrificial gate dielectriclayer 31 includes one or more layers of insulating material, such as asilicon oxide-based material. In one embodiment, silicon oxide formed byCVD is used. The thickness of the sacrificial gate dielectric layer 31is in a range from about 1 nm to about 5 nm in some embodiments.

The sacrificial gate structure 38 is formed by first blanket depositingthe sacrificial gate dielectric layer 31 over the fin structures. Asacrificial gate electrode layer is then blanket deposited on thesacrificial gate dielectric layer and over the fin structures, such thatthe fin structures are fully embedded in the sacrificial gate electrodelayer. The sacrificial gate electrode layer includes silicon such aspolycrystalline silicon or amorphous silicon. The thickness of thesacrificial gate electrode layer is in a range from about 100 nm toabout 200 nm in some embodiments. In some embodiments, the sacrificialgate electrode layer is subjected to a planarization operation. Thesacrificial gate dielectric layer and the sacrificial gate electrodelayer are deposited using CVD, including LPCVD and PECVD, PVD, ALD, orother suitable process. Subsequently, a mask layer is formed over thesacrificial gate electrode layer. The mask layer includes a pad SiNlayer and a silicon oxide mask layer in some embodiments.

Next, a patterning operation is performed on the mask layer andsacrificial gate electrode layer is patterned into the sacrificial gateelectrode 30, as shown in FIGS. 9A and 9B. The sacrificial gatestructure includes the sacrificial gate dielectric layer 31, thesacrificial gate electrode layer 30 (e.g., poly silicon), the pad SiNlayer and the silicon oxide mask layer (not shown). In some embodiments,the source/drain regions of the fin structures are covered by thesacrificial gate dielectric layer 31. In FIGS. 9A-9C, one sacrificialgate structure is formed, but the number of the sacrificial gatestructures is not limited to one. Two or more sacrificial gatestructures are arranged in the X direction in some embodiments. Incertain embodiments, one or more dummy sacrificial gate structures areformed on both sides of the sacrificial gate structures to improvepattern fidelity.

Then, the sacrificial gate dielectric layer 31 covering the source/drainregions of the fin structures is removed by dry and/or wet etching, asshown in FIGS. 10A-10C. By this operation, the stacked layers of thefirst and second semiconductor layers are partially exposed on oppositesides of the sacrificial gate structure 38.

Further, a cover layer for sidewall spacers 32 is conformally formedover the sacrificial gate structure 38, as shown in FIGS. 11A-11C. Thecover layer 32 is deposited in a conformal manner so that it is formedto have substantially equal thicknesses on vertical surfaces, such asthe sidewalls, horizontal surfaces, and the top of the sacrificial gatestructure, respectively. In some embodiments, the cover layer 32 has athickness in a range from about 5 nm to about 20 nm. The cover layer 32includes one or more of SiN, SiON and SiCN or any other suitabledielectric material. The cover layer 32 can be formed by ALD or CVD, orany other suitable method.

Next, as shown in FIGS. 12A-12C, the source/drain regions of the stackedfin structures are etched down below the upper surface of the isolationinsulating layer 15 by using one or more lithography and etchingoperations. One or more etching operations are performed to remove thefin sidewall spacer 32 and then to remove the stacked structures of thefirst and second semiconductor layers and a part of the upper portion ofthe fin structure 11.

Subsequently, as shown in FIGS. 13A-13C, the first semiconductor layers20 are laterally etched in the X direction. The amount of etching of thefirst semiconductor layer 20 is in a range from about 2 nm to about 10nm in some embodiments. When the first semiconductor layers 20 are Ge orSiGe and the second semiconductor layers 25 are Si, the firstsemiconductor layers 20 can be selectively etched by using a wet etchantsuch as, but not limited to, ammonium hydroxide (NH₄OH),tetramethylammonium hydroxide (TMAH), ethylenediamine pyrocatechol(EDP), or potassium hydroxide (KOH) solutions.

Then, as shown in FIGS. 14A-14C, a protective layer is formed to coverthe structures shown by FIGS. 13A-13C. The protective layer includes afirst protective layer 33 made of a silicon oxide based dielectricmaterial (e.g., SiO₂) and a second protective layer 34 made of a siliconnitride based dielectric material (e.g., SiN, SiON). The first andsecond protective layers can be formed through a process such as CVD orALD or any other suitable method. The thickness of the first protectivelayer 33 is in a range from about 1 nm to about 5 nm and the thicknessof the second protective layer 34 is in a range from about 2 nm to about10 nm in some embodiments.

After the protective layers are formed, anisotropic etching is performedto remove the protective layers at least from the upper portion of thefin structure 11, while the protective layers cover the lateral ends ofthe first and second semiconductor layers, as shown in FIGS. 15A-15C.

Then, an additional dry etching is performed on the exposed upperportion of the fin structure to deepen the depth of the groove 18, asshown in FIGS. 16A-16C. The depth D1 of the groove measured from theupper surface of the isolation insulating layer 15 is in a range fromabout 5 nm to about 50 nm in some embodiments. Since the upper portionof the fin structure at the source/drain regions is subjected to atleast two dry etching operations, the groove 18 has a step at the middleof the groove 18, as shown in FIGS. 1D and 1E.

After the groove 18 is formed, the second protective layer 34 and thefirst protective layer 33 are removed by appropriate etching operations,as shown in FIGS. 17A-17C.

Then, a dielectric layer 35 is formed, as shown in FIGS. 18A-18C, tofill the groove 18 and to cover the surrounding portions and the lateralends of the first and second semiconductor layers. In particular, thedielectric layer 35 is formed on the lateral ends of the firstsemiconductor layers, which have been laterally etched. The dielectriclayer 35 is made of suitable dielectric materials such as silicon oxide,silicon oxynitride, silicon nitride, fluorine-doped silicate glass(FSG), low-k dielectrics such as carbon doped oxides (SiOC, SiOCN),extremely low-k dielectrics such as porous carbon doped silicon dioxide,a polymer such as polyimide, combinations of these, or the like. In someembodiments, the dielectric layer 35 includes one or more layers oflow-k dielectric material. The dielectric layer 35 can be formed througha process such as CVD, flowable CVD (FCVD), ALD, or a spin-on-glassprocess, although any acceptable process may be utilized.

In some embodiments, before forming the dielectric layer 35, aninsulating layer is conformally formed on the lateral ends of the firstsemiconductor layer 20 and on the second semiconductor layer 25. Theinsulating layer functions as an etch-stop layer in the subsequentchannel formation operations. The insulating layer includes one ofsilicon nitride (SiN) and silicon oxide (SiO₂), and has a thickness in arange from about 0.5 nm to about 3.0 nm. In other embodiments, theinsulating layer has a thickness in a range from about 1.0 nm to about2.0 nm. The insulating layer can be formed by ALD or any other suitablemethod.

After the dielectric layer 35 is formed, one or more etching operationsare performed to remove unnecessary portion of the dielectric layer 35as shown in FIGS. 19A-19C. By this etching operation, inner spacers areformed on the lateral ends of the first semiconductor layers 20, whichhave been laterally etched, and dielectric spacers are formed in thegrooves 18 and on the surface of surrounding portion of the isolationinsulating layer 15. Further, by this etching, the lateral ends of thesecond semiconductor layer 25 are exposed.

Subsequently, a source/drain epitaxial layer 50 is formed, as shown inFIGS. 20A-20C. The source/drain epitaxial layer 50 includes one or morelayers of Si, SiP, SiC and SiCP for an n-channel FET. The source/drainepitaxial layer 50 is formed by an epitaxial growth method using CVD,ALD or molecular beam epitaxy (MBE). As shown in FIGS. 20A-20C, thesource/drain epitaxial layer 50 is formed individually above eachcorresponding to fin structure 11 in some embodiments. In otherembodiments, as shown in FIG. 20D, the adjacent source/drain epitaxiallayers 50 merge to form a merged source/drain epitaxial layer 50 with avoid 51.

As shown in FIGS. 20A-20D, a bottom of the source/drain epitaxial layer50 is separated from the fin structure 11 by the dielectric layer 35.Further, the inner spacers 35 made of the same material as thedielectric layer 35 are disposed between the source/drain epitaxiallayer 50 and the lateral ends of the first semiconductor layer 20.

FIGS. 21-24 show operations for manufacturing a metal gate structure bya gate-replacement technology. In FIGS. 21-24, a merged source/drainstructure similar to FIG. 20D is employed for the explanation purpose,but the individual source/drain epitaxial layer structure as shown inFIGS. 20A-20C can be applied in the operations of FIGS. 21-24.

After the source/drain epitaxial layer 50 is formed, a contact etch stoplayer (CESL) 60 is formed over the isolation insulating layer 15,sidewall spacers 32 and the source/drain epitaxial layer 50, and aninterlayer dielectric (ILD) layer 80 is formed over the CESL 60, asshown in FIG. 21. The CESL 60 is made of SiN or a silicon nitride-basedmaterial (e.g., SiON, SiCN or SiOCN). The materials for the ILD layer 80include compounds comprising Si, O, C and/or H, such as silicon oxide,SiCOH and SiOC. Organic materials, such as polymers, may be used for theILD layer 80. The CESL 60 can be formed by CVD, ALD or any othersuitable film formation methods. The ILD layer 80 can be formed by CVD,flowable CVD or any other suitable film formation methods. An annealoperation may be performed after the formation of the ILD layer. Afterthe ILD layer 80 is formed, a planarization operation, such as CMP, isperformed, so that the top portion of the sacrificial gate electrodelayer 30 is exposed.

Then, the sacrificial gate structure 38 including the sacrificialelectrode layer 30 and sacrificial gate dielectric layer 31 are removed,thereby forming a gate space 39, as shown in FIG. 22. The ILD layer 80protects the source/drain epitaxial layer 50 during the removal of thesacrificial gate structures. The sacrificial gate structures can beremoved using plasma dry etching and/or wet etching. When thesacrificial gate electrode layer 30 is polysilicon and the ILD layer 36is silicon oxide, a wet etchant such as a TMAH solution can be used toselectively remove the sacrificial gate electrode layer 30. Thesacrificial gate dielectric layer 31 is thereafter removed using plasmadry etching and/or wet etching.

After the sacrificial gate structures are removed, the firstsemiconductor layers 20 are removed, thereby forming wires of the secondsemiconductor layers 25, as channel regions, as shown in FIG. 23. Thefirst semiconductor layers 20 can be removed or etched using an etchantthat can selectively etch the first semiconductor layers 20 against thesecond semiconductor layers 25, as set forth above. In some embodiments,when the insulating layer is formed before the dielectric layer 35 isformed, the etching of the first semiconductor layers 20 stops at theinsulating layer.

After the semiconductor wires of the second semiconductor layers 25 areformed, a gate dielectric layer 102 is formed around each channel layers(wires of the second semiconductor layers 25), and a gate electrodelayer 104 is formed on the gate dielectric layer 102, as shown in FIG.24.

In some embodiments, the gate dielectric layer 102 includes aninterfacial layer 102A and a high-k dielectric layer 102B (see, FIGS.1A-1C). The interfacial layer 102A is a chemically formed silicon oxidein some embodiments. In certain embodiments, the high-k gate dielectriclayer 102B includes one or more layers of a dielectric material, such asHfO₂, HfSiO, HfSiON, HfTaO, HfTiO, HfZrO, zirconium oxide, aluminumoxide, titanium oxide, hafnium dioxide-alumina (HfO₂—Al₂O₃) alloy, othersuitable high-k dielectric materials, and/or combinations thereof.

The high-k gate dielectric layer 102B can be formed by CVD, ALD or anysuitable method. In one embodiment, the high-k gate dielectric layer102B is formed using a highly conformal deposition process such as ALDin order to ensure the formation of a gate dielectric layer having auniform thickness around each channel layers. The thickness of thehigh-k gate dielectric layer 102B is in a range from about 1 nm to about6 nm in one embodiment.

The gate electrode layer 104 (a body gate electrode layer) is formed onthe gate dielectric layer 102 to surround each channel layer. The gateelectrode layer 103 includes one or more layers of conductive material,such as polysilicon, aluminum, copper, titanium, tantalum, tungsten,cobalt, molybdenum, tantalum nitride, nickel silicide, cobalt silicide,TiN, WN, TiAl, TiAlN, TaCN, TaC, TaSiN, metal alloys, other suitablematerials, and/or combinations thereof.

The gate electrode layer 104 may be formed by CVD, ALD, electro-plating,or other suitable method. The gate electrode layer is also depositedover the upper surface of the ILD layer 80. The gate dielectric layerand the gate electrode layer formed over the ILD layer 80 are thenplanarized by using, for example, CMP, until the top surface of the ILDlayer 80 is revealed. In some embodiments, after the planarizationoperation, the gate electrode layer 104 is recessed and a cap insulatinglayer 106 is formed over the recessed gate electrode layer 104. The capinsulating layer includes one or more layers of a silicon nitride-basedmaterial, such as SiN. The cap insulating layer 106 can be formed bydepositing an insulating material followed by a planarization operation.

In certain embodiments, one or more work function adjustment layers 104Aand 104B are interposed between the gate dielectric layer 102 and a bodygate electrode layer 104C. The work function adjustment layers are madeof a conductive material such as a single layer of TiN, TaN, TaAlC, TiC,TaC, Co, Al, TiAl, HfTi, TiSi, TaSi or TiAlC, or a multilayer of two ormore of these materials. For the n-channel FET, one or more of TaN,TaAlC, TiN, TiC, Co, TiAl, HfTi, TiSi and TaSi is used as the workfunction adjustment layer. The work function adjustment layers may beformed by ALD, PVD, CVD, e-beam evaporation, or other suitable process.

By these operations, the structure shown in FIGS. 1A-1E are obtained. Itis understood that the GAA FETs undergoes further CMOS processes to formvarious features such as contacts/vias, interconnect metal layers,dielectric layers, passivation layers, etc.

FIGS. 25A-30C show various stages of manufacturing a GAA FET deviceaccording to another embodiment of the present disclosure. It isunderstood that additional operations can be provided before, during,and after processes shown by FIGS. 25A-30C, and some of the operationsdescribed below can be replaced or eliminated, for additionalembodiments of the method. The order of the operations/processes may beinterchangeable. Material, configuration, dimensions and/or processesthe same as or similar to the foregoing embodiments described withrespect to FIGS. 1A-24 may be employed in the embodiment of FIGS.25A-30C, and detailed explanation thereof may be omitted. In FIGS.25A-30C, the “A” figures (FIGS. 25A, 26A, . . . ) are perspective views,the “B” figures (FIGS. 25B, 26B, . . . ) are cross sectional viewscutting a channel region along the X direction, and the “C” figures(FIGS. 25C, 25C, . . . ) are cross sectional views at a source/drainregion along the Y direction. In this embodiment, a manufacturingoperation for an n-type GAA FET will be explained.

FIGS. 25A-25C are the same as FIGS. 15A-15C as set forth above. In thisembodiment, the groove 18 is formed by using wet etching, as shown inFIGS. 26A-26C. When the fin structure 11 is made of Si,tetramethylammonium hydroxide (TMAH) can be used as a wet-etchant. SinceTMAH anisotropically etches crystalline Si, the sidewalls of the groove18 have a (111) facet of Si crystal, as shown in FIGS. 2D and 2E. Thedepth D2 of the groove measured from the upper surface of the isolationinsulating layer 15 is in a range from about 5 nm to about 50 nm in someembodiments.

After the groove 18 is formed by wet etching, the second protectivelayer 34 and the first protective layer 33 are removed by appropriateetching operations, as shown in FIGS. 27A-27C. The operations to removethe protective layers are substantially the same as those explained withrespect to FIGS. 17A-17C.

Then, similar to FIGS. 18A-18C, a dielectric layer 35 is formed, asshown in FIGS. 28A-28C, to fill the groove 18 and to cover thesurrounding portions and the lateral ends of the first and secondsemiconductor layers. The operations to form the dielectric layer 35 aresubstantially the same as those explained with respect to FIGS. 18A-18C.

After the dielectric layer 35 is formed, one or more etching operationsare performed to remove unnecessary portions of the dielectric layer 35as shown in FIGS. 29A-29C. The operations to partially etch thedielectric layer 35 are substantially the same as those explained withrespect to FIGS. 19A-19C. By this etching operation, inner spacers areformed on the lateral ends of the first semiconductor layers 20, whichhave been laterally etched, and dielectric spacers are formed in thegrooves 18 and on the surface of surrounding portion of the isolationinsulating layer 15. Further, by this etching, the lateral ends of thesecond semiconductor layer 25 are exposed.

Subsequently, a source/drain epitaxial layer 50 is formed, as shown inFIGS. 30A-30C. The operations to form the source/drain epitaxial layer50 are substantially the same as those explained with respect to FIGS.20A-20C.

After the source/drain epitaxial layer 50 is formed, the same as orsimilar operations explained with respect to FIGS. 21-24 are performed,and the structure shown in FIGS. 2A-2E are obtained. It is understoodthat the GAA FETs undergoes further CMOS processes to form variousfeatures such as contacts/vias, interconnect metal layers, dielectriclayers, passivation layers, etc.

FIGS. 31A-38C show various stages of manufacturing a GAA FET deviceaccording to another embodiment of the present disclosure. It isunderstood that additional operations can be provided before, during,and after processes shown by FIGS. 31A-38C, and some of the operationsdescribed below can be replaced or eliminated, for additionalembodiments of the method. The order of the operations/processes may beinterchangeable. Material, configuration, dimensions and/or processesthe same as or similar to the foregoing embodiments described withrespect to FIGS. 1A-30C may be employed in the embodiment of FIGS.31A-38C, and detailed explanation thereof may be omitted. In FIGS.31A-38C, the “A” figures (FIGS. 31A, 32A, . . . ) are perspective views,the “B” figures (FIGS. 31B, 32B, . . . ) are cross sectional viewscutting a channel region along the X direction, and the “C” figures(FIGS. 31C, 32C, . . . ) are cross sectional views at a source/drainregion along the Y direction. In this embodiment, a manufacturingoperation for a p-type GAA FET will be explained.

After the structure shown in FIGS. 12A-12C is formed, the secondsemiconductor layers 25 are laterally etched in the X direction, asshown in FIGS. 31A-31C. The amount of etching of the secondsemiconductor layer 25 is in a range from about 2 nm to about 10 nm insome embodiments. When the first semiconductor layers 20 are Ge or SiGeand the second semiconductor layers 25 are Si, the second semiconductorlayers 25 can be selectively etched by using a wet etchant such as, butnot limited to, ammonium hydroxide (NH₄OH), tetramethylammoniumhydroxide (TMAH), ethylenediamine pyrocatechol (EDP), or potassiumhydroxide (KOH) solutions.

Then, as shown in FIGS. 32A-32C, a protective layer formed to cover thestructures shown by FIGS. 30A-39C. The operations to form the protectivelayer are substantially the same as or similar to those explained withrespect to FIGS. 14A-14C.

After the protective layers are formed, anisotropic etching is performedto remove the protective layers at least from the upper portion of thefin structure 11, while the protective layers cover the lateral ends ofthe first and second semiconductor layers, as shown in FIGS. 33A-33C,similar to FIGS. 15A-15C.

Then, an additional dry etching is performed on the exposed upperportion of the fin structure to deepen the depth of the groove 18, asshown in FIGS. 34A-34C. The depth D3 of the groove measured from theupper surface of the isolation insulating layer 15 is in a range fromabout 5 nm to about 50 nm in some embodiments. Since the upper portionof the fin structure at the source/drain regions is subjected to atleast two dry etching operations, the groove 18 has a step at the middleof the groove 18, as shown in FIG. 3D.

After the groove 18 is formed by wet etching, the second protectivelayer 34 and the first protective layer 33 are removed by appropriateetching operations, as shown in FIGS. 35A-35C. The operations to removethe protective layers are substantially the same as those explained withrespect to FIGS. 17A-17C.

Then, similar to FIGS. 18A-18C, a dielectric layer 35 is formed, asshown in FIGS. 36A-36C, to fill the groove 18 and to cover thesurrounding portions and the lateral ends of the first and secondsemiconductor layers. The operations to form the dielectric layer 35 aresubstantially the same as those explained with respect to FIGS. 18A-18C.

After the dielectric layer 35 is formed, one or more etching operationsare performed to remove unnecessary portion of the dielectric layer 35as shown in FIGS. 37A-37C. The operations to partially etch thedielectric layer 35 are substantially the same as those explained withrespect to FIGS. 19A-19C. By this etching operation, inner spacers areformed on the lateral ends of the second semiconductor layers 25, whichhave been laterally etched, and dielectric spacers are formed in thegrooves 18 and on the surface of surrounding portion of the isolationinsulating layer 15. Further, by this etching, the lateral ends of thefirst semiconductor layer 20 are exposed.

Subsequently, a source/drain epitaxial layer 50 is formed, as shown inFIGS. 38A-38C. The source/drain epitaxial layer 50 includes one or morelayers of Si, SiGe or Ge for a p-channel FET. The operations to form thesource/drain epitaxial layer 50 are substantially the same as thoseexplained with respect to FIGS. 20A-20C.

After the source/drain epitaxial layer 50 is formed, the same as orsimilar operations explained with respect to FIGS. 21-24 are performed,and the structure shown in FIGS. 3A-3D are obtained. In the channelformation process explained with respect to FIG. 23, the secondsemiconductor layers 25 are removed, thereby leaving the firstsemiconductor layer as channel regions of the GAA FET. Further, in thisprocess, the upper portion of the fin structure 11 below the channelregions is also partially etched, and the gate dielectric layer and thegate electrode layer fill the space formed above the fin structure 11and the bottom-most first semiconductor layer 20 (see, FIGS. 3A and 3B).Further, the one or more work function adjustment layers 104A and 104Binclude one or more of TiAlC, Al, TiAl, TaN, TaAlC, TiN, TiC and Co, orany other suitable conductive material.

It is understood that the GAA FETs undergo further CMOS processes toform various features such as contacts/vias, interconnect metal layers,dielectric layers, passivation layers, etc.

FIGS. 39A-43C show various stages of manufacturing a GAA FET deviceaccording to another embodiment of the present disclosure. It isunderstood that additional operations can be provided before, during,and after processes shown by FIGS. 39A-43C, and some of the operationsdescribed below can be replaced or eliminated, for additionalembodiments of the method. The order of the operations/processes may beinterchangeable. Material, configuration, dimensions and/or processesthe same as or similar to the foregoing embodiments described withrespect to FIGS. 1A-38C may be employed in the embodiment of FIGS.39A-43C, and detailed explanation thereof may be omitted. In FIGS.39A-43C, the “A” figures (FIGS. 39A, 40A, . . . ) are perspective views,the “B” figures (FIGS. 39B, 40B, . . . ) are cross sectional viewscutting a channel region along the X direction, and the “C” figures(FIGS. 39C, 40C, . . . ) are cross sectional views at a source/drainregion along the Y direction. In this embodiment, a manufacturingoperation for a p-type GAA FET will be explained.

FIGS. 39A-39C are the same as FIGS. 33A-33C.

After the protective layers are removed, the groove 18 is formed byusing wet etching, as shown in FIGS. 40A-40C, similar to FIGS. 26A-26C.When the fin structure 11 is made of Si, tetramethylammonium hydroxide(TMAH) can be used as a wet-etchant. Since TMAH anisotropically etchescrystalline Si, the sidewalls of the groove 18 have a (111) facet of Sicrystal, as shown in FIG. 4D. The depth D4 of the groove measured fromthe upper surface of the isolation insulating layer 15 is in a rangefrom about 5 nm to about 50 nm in some embodiments.

After the groove 18 is formed by wet etching, the second protectivelayer 34 and the first protective layer 33 are removed by appropriateetching operations, as shown in FIGS. 41A-41C. The operations to removethe protective layers are substantially the same as those explained withrespect to FIGS. 17A-17C.

Then, similar to FIGS. 18A-18C, a dielectric layer 35 is formed, asshown in FIGS. 42A-42C, to fill the groove 18 and to cover thesurrounding portions and the lateral ends of the first and secondsemiconductor layers. The operations to form the dielectric layer 35 aresubstantially the same as those explained with respect to FIGS. 18A-18C.

After the dielectric layer 35 is formed, one or more etching operationsare performed to remove unnecessary portion of the dielectric layer 35as shown in FIGS. 43A-43C. The operations to partially etch thedielectric layer 35 are substantially the same as those explained withrespect to FIGS. 19A-19C. By this etching operation, inner spacers areformed on the lateral ends of the second semiconductor layers 25, whichhave been laterally etched, and dielectric spacers are formed in thegrooves 18 and on the surface of surrounding portion of the isolationinsulating layer 15. Further, by this etching, the lateral ends of thefirst semiconductor layer 20 are exposed.

Subsequently, a source/drain epitaxial layer 50 is formed, as shown inFIGS. 44A-44C. The source/drain epitaxial layer 50 includes one or morelayers of Si, SiGe or Ge for a p-channel FET. The operations to form thesource/drain epitaxial layer 50 are substantially the same as thoseexplained with respect to FIGS. 20A-20C.

After the source/drain epitaxial layer 50 is formed, the same as orsimilar operations explained with respect to FIGS. 21-24 are performed,and the structure shown in FIGS. 4A-4D are obtained. In the channelformation process explained with respect to FIG. 23, the secondsemiconductor layers 25 are removed, thereby leaving the firstsemiconductor layer as channel regions of the GAA FET. Further, in thisprocess, the upper portion of the fin structure 11 below the channelregions is also partially etched, and the gate dielectric layer and thegate electrode layer fill the space formed above the fin structure 11and the bottom-most first semiconductor layer 20 (see, FIGS. 4A and 4B).Further, the one or more work function adjustment layers 104A and 104Binclude one or more of TiAlC, Al, TiAl, TaN, TaAlC, TiN, TiC and Co, orany other suitable conductive material.

It is understood that the GAA FETs undergo further CMOS processes toform various features such as contacts/vias, interconnect metal layers,dielectric layers, passivation layers, etc.

The various embodiments or examples described herein offer severaladvantages over the existing art. For example, in the presentdisclosure, since a dielectric layer is inserted between the bottom ofthe source/drain epitaxial layer and the fin structure, it is possibleto reduce an off-state leakage current with reducing an amount of APTdose. Further, the inner spacers and the dielectric layer are formed bythe same operations, the process to form the inner spacers can beeasier.

It will be understood that not all advantages have been necessarilydiscussed herein, no particular advantage is required for allembodiments or examples, and other embodiments or examples may offerdifferent advantages.

In accordance with an aspect of the present disclosure, in a method ofmanufacturing a semiconductor device, a fin structure, in which firstsemiconductor layers and second semiconductor layers are alternatelystacked, is formed. A sacrificial gate structure is formed over the finstructure. The first semiconductor layers, the second semiconductorlayers and an upper portion of the fin structure at a source/drainregion of the fin structure, which is not covered by the sacrificialgate structure, are etched. A dielectric layer is formed over the etchedupper portion of the fin structure. A source/drain epitaxial layer isformed. The source/drain epitaxial layer is connected to ends of thesecond semiconductor layers, and a bottom of the source/drain epitaxiallayer is separated from the fin structure by the dielectric layer. Inone or more of the foregoing and following embodiments, after the firstsemiconductor layers, the second semiconductor layers and the upperportion of the fin structure are etched, the first semiconductor layersare laterally etched. In one or more of the foregoing and followingembodiments, the dielectric layer is also formed on laterally etchedends of the first semiconductor layers. In one or more of the foregoingand following embodiments, after the first semiconductor layers, thesecond semiconductor layers and the upper portion of the fin structureare etched, the etched upper portion of the fin structure is furtheretched, while the first and second semiconductor layers are protected bya cover layer. In one or more of the foregoing and followingembodiments, the etched upper portion of the fin structure is etched bywet etching. In one or more of the foregoing and following embodiments,after the etched upper portion of the fin structure is etched and beforethe dielectric layer is formed, the cover layer is removed. In one ormore of the foregoing and following embodiments, after the source/drainepitaxial layer is formed, an interlayer dielectric (ILD) layer isformed, the sacrificial gate structure are removed, thereby exposing apart of the fin structure, the first semiconductor layers are removedfrom the exposed fin structure, thereby forming channel layers includingthe second semiconductor layers, and a gate dielectric layer and a gateelectrode layer are formed around the channel layers. In one or more ofthe foregoing and following embodiments, a bottom portion of the finstructure is embedded an isolation insulating layer, and the dielectriclayer is made of a different material than the isolation insulatinglayer and the ILD layer. In one or more of the foregoing and followingembodiments, the first semiconductor layers are made of Si_(1-x)Ge_(x)and the second semiconductor layers are made of Si_(1-y)Ge_(y), where0≤x<y<1. In one or more of the foregoing and following embodiments, thefirst semiconductor layers are made of Si_(1-x)Ge_(x), where 0.2≤x≤0.8,and the second semiconductor layers are made of Si. In one or more ofthe foregoing and following embodiments, the dielectric layer is made ofSiCO or SiOCN. In one or more of the foregoing and followingembodiments, the dielectric layer is made of silicon oxide or siliconnitride.

In accordance with another aspect of the present disclosure, in a methodof manufacturing a semiconductor device, a fin structure, in which firstsemiconductor layers and second semiconductor layers are alternatelystacked, is formed. A sacrificial gate structure is formed over the finstructure. A sidewall spacer is formed on a side of the sacrificial gatestructure. The first semiconductor layers, the second semiconductorlayers and an upper portion of the fin structure, which are not coveredby the sacrificial gate structure and the sidewall spacer are etched. Acover layer is formed to cover the first semiconductor layers and thesecond semiconductor layers. The upper portion of the fin structure areetched. A dielectric layer is formed over the etched upper portion ofthe fin structure. A source/drain epitaxial layer is formed. In one ormore of the foregoing and following embodiments, the cover layerincludes a silicon oxide layer and a silicon nitride layer. In one ormore of the foregoing and following embodiments, after the firstsemiconductor layers, the second semiconductor layer and the upperportion of the fin structure are etched and before the cover layer isformed, the first semiconductor layers under the sidewall spacer arelaterally etched. In one or more of the foregoing and followingembodiments, after the etched upper portion of the fin structure isfurther etched and before the dielectric layer is formed, the coverlayer is removed. In one or more of the foregoing and followingembodiments, the etched upper portion of the fin structure is etched bywet etching.

In accordance with another aspect of the present disclosure, in a methodof manufacturing a semiconductor device, a first fin structure and asecond fin structure, in both of which first semiconductor layers andsecond semiconductor layers are alternately stacked, are formed. Asacrificial gate structure is formed over the first and second finstructure. The first semiconductor layers, the second semiconductorlayers and an upper portion of the fin structure at source/drain regionsof the first and second fin structures, which are not covered by thesacrificial gate structure, are etched. A dielectric layer is formedover the etched upper portions of the first and second fin structures. Afirst source/drain epitaxial layer is formed over the first finstructure and a second source/drain epitaxial layer over the second finstructure. A bottom of the first source/drain epitaxial layer isseparated from the first fin structure by the dielectric layer, and abottom of the second source/drain epitaxial layer is separated from thesecond fin structure by the dielectric layer. In one or more of theforegoing and following embodiments, the first source/drain epitaxiallayer is separated from the second source/drain epitaxial layer. In oneor more of the foregoing and following embodiments, the firstsource/drain epitaxial layer is connected to the second source/drainepitaxial layer.

In accordance with one aspect of the present disclosure, a semiconductordevice includes semiconductor wires vertically arranged, each of whichhas a channel region, and a source/drain epitaxial layer connected toends of the semiconductor wires. The semiconductor wires and thesource/drain epitaxial layer are disposed above a fin structure, and abottom of the source/drain epitaxial layer is separated from the finstructure by a dielectric layer. In one or more of the foregoing andfollowing embodiments, the semiconductor device further includes anisolation insulating layer in which the fin structure is embedded, andan interlayer dielectric (ILD) layer covering the source/drain epitaxiallayer. The dielectric layer is made of a different material than theisolation insulating layer and the ILD layer. In one or more of theforegoing and following embodiments, the dielectric layer is made ofSiCO or SiOCN. In one or more of the foregoing and followingembodiments, the dielectric layer is in contact with the source/drainepitaxial layer and the fin structure. In one or more of the foregoingand following embodiments, an upper surface of the fin structure incontact with the dielectric layer has a V-shaped groove. In one or moreof the foregoing and following embodiments, the fin structure is made ofSi, and the V-shaped groove includes a (111) crystal facet of Si. In oneor more of the foregoing and following embodiments, the semiconductordevice further includes a gate structure wrapping around the channelregion of each of the semiconductor wires, and inner spacers disposedbetween the source/drain epitaxial layer and portions of the gatestructure arranged between adjacent semiconductor wires. In one or moreof the foregoing and following embodiments, the inner spacers are madeof a same material as the dielectric layer. In one or more of theforegoing and following embodiments, the semiconductor wires are made ofSi. In one or more of the foregoing and following embodiments, thesemiconductor wires are made of SiGe and the fin structures are made ofSi.

In accordance with another aspect of the present disclosure, asemiconductor device includes a first group of semiconductor wiresvertically arranged above a first fin structure, each of which has achannel region, a first source/drain epitaxial layer connected to endsof the semiconductor wires of the first group and disposed above thefirst fin structure, a second group of semiconductor wires verticallyarranged above a second fin structure, each of which has a channelregion, and a second source/drain epitaxial layer connected to ends ofthe semiconductor wires of the second group and disposed above thesecond fin structure. The first fin structure is adjacent to the secondfin structure with an isolation insulating layer interposed between thefirst and second fin structures. A bottom of the first source/drainepitaxial layer is separated from the first fin structure by adielectric layer, and a bottom of the second source/drain epitaxiallayer is separated from the second fin structure by the dielectriclayer. In one or more of the foregoing and following embodiments, thefirst source/drain epitaxial layer is separated from the secondsource/drain epitaxial layer. In one or more of the foregoing andfollowing embodiments, the first source/drain epitaxial layer isconnected to the second source/drain epitaxial layer. In one or more ofthe foregoing and following embodiments, the semiconductor devicefurther includes an interlayer dielectric (ILD) layer covering thesource/drain epitaxial layer. The dielectric layer is made of adifferent material than the isolation insulating layer and the ILDlayer. In one or more of the foregoing and following embodiments, thedielectric layer is made of SiCO or SiOCN. In one or more of theforegoing and following embodiments, the dielectric layer is made ofsilicon oxide or silicon nitride.

In accordance with another aspect of the present disclosure, asemiconductor device includes semiconductor wires vertically arrangedand extending in a first direction, each of which has a channel region,and a source/drain epitaxial layer connected to ends of thesemiconductor wires. The semiconductor wires and the source/drain regionare disposed above a fin structure. A bottom of the source/drainepitaxial layer is separated from the fin structure by a dielectriclayer. The dielectric layer is in contact with the fin structure, and anupper surface of the fin structure in contact with the dielectric layerhas a groove. In one or more of the foregoing and following embodiments,in a cross section defined by the first direction and the verticaldirection, the groove has a middle portion of which width is larger thanwidths of a bottom portion and upper portion of the groove. In one ormore of the foregoing and following embodiments, in a cross sectiondefined by the first direction and the vertical direction, a width ofthe groove, from a bottom to a top, increases, decreases, increases andthen decreases. In one or more of the foregoing and followingembodiments, the groove includes a (111) crystal facet of Si.

The foregoing outlines features of several embodiments or examples sothat those skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art should appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodiments orexamples introduced herein. Those skilled in the art should also realizethat such equivalent constructions do not depart from the spirit andscope of the present disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A method of manufacturing a semiconductor device,comprising: forming a fin structure in which first semiconductor layersand second semiconductor layers are alternately stacked over a bottomfin structure; forming a sacrificial gate structure over the finstructure; removing by etching the first semiconductor layers, thesecond semiconductor layers and an upper portion of the bottom finstructure at a source/drain region of the fin structure, which is notcovered by the sacrificial gate structure; forming a dielectric layerover the etched upper portion of the bottom fin structure by using adeposition operation and an etching operation; and forming asource/drain epitaxial layer, wherein: the source/drain epitaxial layeris connected to ends of the second semiconductor layers, a bottom of thesource/drain epitaxial layer is separated from the bottom fin structureby the dielectric layer, after the first semiconductor layers, thesecond semiconductor layers and the upper portion of the bottom finstructure are etched, laterally etching the first semiconductor layers,and the dielectric layer is also formed on laterally etched ends of thefirst semiconductor layers.
 2. The method of claim 1, wherein, after thefirst semiconductor layers, the second semiconductor layers and theupper portion of the bottom fin structure are etched, the etched upperportion of the bottom fin structure is further etched, while the firstand second semiconductor layers are protected by a cover layer.
 3. Themethod of claim 2, wherein the etched upper portion of the bottom finstructure is etched by wet etching.
 4. The method of claim 2, whereinafter the etched upper portion of the bottom fin structure is furtheretched and before the dielectric layer is formed, the cover layer isremoved.
 5. The method of claim 1, further comprising, after thesource/drain epitaxial layer is formed: forming an interlayer dielectric(ILD) layer; removing the sacrificial gate structure, thereby exposing apart of the fin structure; removing the first semiconductor layers fromthe exposed fin structure, thereby forming channel layers including thesecond semiconductor layers; and forming a gate dielectric layer and agate electrode layer around the channel layers.
 6. The method of claim5, wherein: the bottom fin structure is embedded an isolation insulatinglayer, and the dielectric layer is made of a different material than theisolation insulating layer and the ILD layer.
 7. The method of claim 5,wherein the first semiconductor layers are made of Si_(1-x)Ge_(x) andthe second semiconductor layers are made of Si_(1-y)Ge_(y), where0≤x<y<1.
 8. The method of claim 5, wherein the first semiconductorlayers are made of Si_(1-x)Ge_(x), where 0.2≤x≤0.8, and the secondsemiconductor layers are made of Si.
 9. The method of claim 1, whereinthe dielectric layer is made of SiCO or SiOCN.
 10. The method of claim1, wherein the dielectric layer is made of silicon oxide or siliconnitride.
 11. A method of manufacturing a semiconductor device,comprising: forming a fin structure in which first semiconductor layersand second semiconductor layers are alternately stacked over a bottomfin structure; forming a sacrificial gate structure over the finstructure; forming a sidewall spacer on a side of the sacrificial gatestructure; etching the first semiconductor layers, the secondsemiconductor layer and an upper portion of the bottom fin structure,which are not covered by the sacrificial gate structure and the sidewallspacer; forming a cover layer to cover the first semiconductor layersand the second semiconductor layers; etching the upper portion of thebottom fin structure; forming a dielectric layer over the etched upperportion of the bottom fin structure by using a deposition operation andan etching operation; and forming a source/drain epitaxial layer,wherein the method further comprises, after the first semiconductorlayers, the second semiconductor layer and the upper portion of thebottom fin structure are etched and before the cover layer is formed,laterally etching the first semiconductor layers under the sidewallspacer.
 12. The method of claim 11, wherein the cover layer includes asilicon oxide layer and a silicon nitride layer.
 13. The method of claim11, wherein after the etched upper portion of the bottom fin structureis further etched and before the dielectric layer is formed, the coverlayer is removed.
 14. The method of claim 11, wherein the etched upperportion of the bottom fin structure is etched by wet etching.